This invention relates to an electrolytic cell for processes in which gas is evolved.
It is known that in the electrolysis of alkali metal chloride the electrode voltage at the gas-producing electrodes exceeds the voltage which corresponds to the thermodynamic equilibrium conditions. This phenomenon accounts for part of the overvoltage and is due to the fact that the gas bubbles formed during the electrolysis cover a part of the electrode surface and block said part of the surface for a flow of current. For this reason, a correspondingly higher current flows through adjacent electrode portions, when the total current is given. This partial increase in current density results necessarily in a voltage rise in this area and this voltage rise is virtually quantitatively transformed into heat and causes a temperature rise of the electrode surface. Because the gas cushion on the electrode surface opposes a rapid heat exchange with the electrolyte, the dissipation of said heat is relatively poor. The eventually resulting temperatures in infinitesimal areas of the electrode surfaces are far in excess of 100.degree. C in commercial electrolytic processes and are responsible, inter alia, for corrosion phenomena on the electrodes.
Numerous proposals have been made to reduce the economically undesirable overvoltage and to restrict the attack of the electrode surface. For instance, the anode has been provided with a multiplicity of cylindrical holes or slots, which serve to discharge as quickly as possible the chlorine gas that has been evolved (Published German Applications 1,667,812; 1,792,183; British Pat. No. 1,229,402). Gas flow areas on an order of 15 - 35% of the total area of the anode are usual in such cases. Larger gas flow areas are avoided because they would result in an excessively high effective current density and activation overvoltage. The same purpose is served by numerous metal anode structures which have been proposed and consist, e.g., of expanded metal, slotted plates, or mesh structures.
Where the known proposals are adopted, the gas rises to the surface of the electrolyte on the shortest possible path. The potential energy which is contained in the gas owing to the hydrostatic processure of the electrolyte is randomly destroyed in this case or, more properly speaking, random turbulence is produced in the electrolyte. Dispersed gas bubbles are inevitably returned with the brine which flows into the space between the electrodes.
A further development relating to/the design of the flow passages for gas evolved during the electrolysis has been described in German Utility Modely 7,207,894. In accordance therewith, the flow passages are enlarged at least close to the surface of the electrode and toward said surface. Specifically, Venturi-like passages are provided. While this proposal affords some advantages, it cannot entirely avoid a flow of gas from the outlet of a flow passage into the suction range of the liquid which flows into the space between the electrodes so that said gas is entrained by the liquid.
A certain mode of operating mercury electrolytic cells is known, which involves anodes having groovelike recesses on the side facing the mercury and in which the arrangement of the anodes and/or the groovelike recesses is so selected that spaced apart areas are disposed between the anodes and serve for the outflow of chlorine from the space between the electrodes and for the inflow of brine into said space (U.S. Pat. No. 3,951,767). Partitions may be arranged between adjacent anodes having uniform groove-like recesses. Alternatively, anodes may be inserted in which the bottoms of the groovelike recesses of adjacent anodes are inclined in opposite directions from the horizontal.
While the mode of operation previously described has proved satisfactory and low cell voltages which are expected to be maintained over months of operation, a disadvantage arises which resides in that the mode of operation can be used virtually only in new installations on in existing plants which -- for any reason whatever -- are provided with new anodes. As a rule, there will be no adaptation or change of installations which inherently do/not require a shutdown. In addition, anodes with inclined groovelike recesses are somewhat difficult to manufacture.